An image transfer system includes an intermediate transfer member (ITM) to initially receive an image transferred by a plurality of ink jets, as depicted in the diagram of FIG. 1. The ink jets are configured to transfer an aqueous ink onto the surface of the transfer member in a variety of known manners. The aqueous image is partially dried before reaching the transfer roll. A substrate is pinched between the intermediate transfer member and the transfer roll and the ITM releases the ink image onto the substrate in a transfer step. The substrate is conveyed to post-processing components that fix the image onto the substrate. It is understood that the ITM and transfer roll are continuously rotated and that a substrate or substrates are continuously fed through the transfer system and between the ITM and transfer roll. Image transfer systems of the kind generically depicted in FIG. 1 are used in a wide range of machines, such as printers, copiers, facsimile machines, book making machines and the like.
The surface energy of the surface of the ITM controls how well the ink transferred to the ITM surface is retained on the surface and how well the ink image is released from the ITM onto the substrate. The problems of ink retention and release are exacerbated in high-throughput systems where the substrate is fed at high speeds through the image transfer system. A low surface energy is desirable for optimum image transfer from the surface of the ITM to the surface of the substrate. On the other hand, a low surface energy reduces the ability of the aqueous ink to spread on the ITM surface, resulting in a low image quality. Aqueous ink jet imaging on a low surface energy, non-absorbing surface and then optimal release and transfer to the substrate has been very challenging, with no commercially viable solution thus far.
An optimum surface treatment for an ITM must tackle three challenges: 1) wet image quality; 2) image transfer; and 3) print-head management. The first challenge—wet image quality—prefers a high surface energy on the ITM surface which causes the aqueous ink to spread and wet the surface, rather than beading up into discrete droplets. The second challenge—image transfer—prefers that the ink, once partially dried on the ITM, has minimal attraction to the ITM surface so that 100% of the ink is transferred from the ITM to the substrate. Thus, image transfer is optimized by minimizing ITM surface energy. The third challenge relates to how well the print head can be kept clean of dried ink. For resin-based ink, the drying of the ink on the face plate of a print head can render it inoperable. On the other hand, too much moisture can condense on the face plate and cause jetting problems. In addition, some ink jets can be sensitive to high temperatures, typically temperatures above about 70° C.
Various approaches have been investigated to provide a solution that balances all three challenges, including ITM material selection, ink design and auxiliary fluid methods. With respect to ITM material selection, materials that are known to provide optimum release properties include the classes of silicone, fluorosilicone, TEFLON, VITON and certain hybrid materials. These compositions have low surface energy but provide poor wetting. Alternatively, polyurethane and polymide have been used to improve wetting but at the cost of poor ink release properties. Tuning ink compositions to address these challenges has proven to be very difficult since the primary performance attribute of the ink is the performance in the print head. For instance, if the ink surface tension is too high it will not jet properly and it if is too low it will drool out of the face plate of the print head. Compounding the problem is the fact that for optimal ink transfer, ink cohesion must be significantly greater than the ink-to-ITM adhesion for all image contents, including the stress cases of single layer small dot and three layer process black solid printing.
The wet image quality is directly affected by surface energy of the ITM. As mentioned above, low surface energy is typically necessary for image transfer, but this same low surface energy property diminishes the ability of conventional inks, and more particularly aqueous inks, to spread on the ITM surface. When ink drops coalesce at a state of insufficient spreading, inks from multiple drops can reflow or redistribute in many undesired ways, which ultimately produce image defects. The problem of poor-coalescence is enhanced on non-absorbing substrates because there is no simple mechanism to freeze the motion of the colorant in the ink drops. Examples of the poor wet image quality resulting from low surface energy are shown in FIGS. 2a, 2b. In the example of FIG. 2a, the ink drops forming the figures “c”, “q” and “6” puddle resulting in a very poor image quality. In the example of FIG. 2b, the lines of ink should be 100% continuous at 600 dpi with a 12 pl drop size. However, as the picture illustrates, the ink drops draw back due to poor coalescence to that the lines are sporadic and incomplete, again resulting in a poor image quality. Poor ink spreading is the cause of the poor quality in both examples.
There is a need for an ITM that can maintain a desirably low surface energy density, for image transfer, but can also promote optimum ink spreading.